1. Field of the Invention
The present invention relates to a pneumatic low section tire, and more specifically to an improvement in the tread of a pneumatic low section radial tire.
2. Description of the Prior Art
With increasing vehicle speed, recently tires have a tendency to become low in section; that is, the section height of the tire decreases but the tire width increases. Since the tread width increases with increasing tire width, a preferable type of the low section tires is as follows: 3 to 5 main straight grooves are arranged at regular intervals along the axial direction thereof extending substantially in the circumferential direction of the tread, and connected by transversal grooves having a depth the same as that of the main grooves so as to form blocks, under due consideration of drainage in rainy weather vehicle running. As low section types, radial tires are widely used, because radial structure is suitable for high speed vehicle travel.
In manufacturing a radial tire, a tread rubber with a uniform thickness is extruded and cut off into a predetermined length; the cut-off tread rubber is stuck onto a belt formed by laminating belt layers around a crown portion of a carcass together with beads and side treads, to form a green tire; the green tire is vulcanized or hardened within a mold. In this vulcanization process, a tread pattern including various grooves or blocks are formed by the mold. To form a pattern on the tire tread, various U-shaped skeletal protrusions are arranged in the inner wall of the mold. The height of the skeletal protrusions is about 9 to 16 mm in the case of the ordinary passenger tires, although being different according to the tire size.
When a green tire is pressurized within the mold by applying steam pressure from the inner opening of the tire with a bladder, tread rubber with a relatively high viscosity (although softed a little by heat) is forced to flow into recesses formed between the skeletal protrusions to form tire tread blocks. In a low section tire with an aspect ratio of the section height of the tire to the maximum tire width in cross section of 0.65 or less the tread width increases with increasing tire width and further the green tread rubber is flat with a substantially uniform rubber thickness along the axial (width) direction of the tire. When the tread rubber flows into the mold recesses for forming pattern blocks between two circumferential grooves during vulcanization process, the belt itself is subjected to the influence of the flow of the green rubber and therefore inevitably deformed toward the recesses. When the deformed green tire is vulcanized or hardened as it is, the belt layers rise or are corrugated outward along the longitudinal direction of the transversal grooves formed between two main grooves. The maximum deformation is produced at the middle portion of the transversal groove between two adjacent main grooves, this brings up a problem in that a skid base rubber existing between the groove bottom and the belt layer with a uniform thickness becomes too thin at the middle portion of the transversal groove due to the influence of an outward deformation of the belt layer, thus resulting in premature problems with the tire of low section type.